1. Field of the Invention
The present invention relates to an optical recording medium for optically recording and reproducing information.
2. Related Background Art
Magnetic materials are generally used for conventional recording mediums such as credit cards, floppy disks, and the like. These recording mediums are advantageous in that writing and reading of information can readily be performed, but have problems that the recorded matters can readily be altered and high density recording is not achievable. As means of solving such problems and treating a variety of information efficiently, optical information recording methods were proposed which utilize an optical recording medium, and various proposals therefor have been made regarding the recording mediums, recording-reproducing systems, recording-reproducing apparatuses, and the like.
The optical recording mediums for optical recording and reproduction of information includes those which comprise silver particles dispersed in a gelatin matrix.
Furthermore, a heat-mode recording medium was disclosed in which recording is conducted by projecting a spot-like energy beam such as a laser beam onto the recording layer thereof to change the state of portions of the recording layer. Such a recording medium does not require a developing treatment, allowing direct reading after writing (called a DRAW medium), and enabling high-density recording and additional recording, and is considered to be promising as recording mediums for disks and cards.
Organic coloring matters which change their properties by receiving a relatively long wavelength of light are studied comprehensively for use in optical disks and optical cards, because the organic coloring matters are nontoxic and capable of being formed into a film by a coating method at a lower production cost.
Generally, optical disks have an air gap structure in which a void space is formed at the upper portion of the recording layer to raise recording sensitivity. The formation of the void space in the upper portion of the recording medium facilitates the change of the state of the recording layer to achieve high sensitivity. The optical cards, however, are frequently carried and used in the same manner as in cash cards and credit cards. Therefore, strength against distortion caused by various external forces is necessary. In addition, there is a desire of making the medium thin. Such facts refuse the employ the air gap structure; instead contact-layer structures are generally employed.
Optical disks are preferable to minimize its thickness in view of carriage and storage. As far as the thickness is minimized, the optical disks are also desired to have a contact-layer structure. However, the optical disks having a contact-layer structure disadvantageously exhibit lower recording sensitivity in comparison with the one having the hollow structure.
On the other hand, the formation of a layer of an elastic rubber polymer such as urethane rubber, silicone rubber, and fluororubber on the recording layer was disclosed to improve the recording sensitivity, for example, in Japanese Patent Publication No. 63-42333, and Japanese Patent Application Laid-Open Nos. 58-125248 and 58-203095.
Further, formation of a thermoplastic soft layer of such material as a styrene-butadiene copolymer, a nylon resin, and a polyvinylbutyral resin was disclosed in Japanese Utility Model Application Laid-Open No. 63-2223 to improve the recording sensitivity.
As mentioned above, a record-assisting layer provided on the recording layer is known to improve the recording sensitivity. The material for constituting the record-assisting layer is desired to be capable of forming a film by a coating method on the recording layer without dissolving the recording layer. In the case where the recording layer contains an organic coloring matter, the preferred material for the record-assisting layer includes curable rubber type polymer which exhibits rubber elasticity after curing: for example, silicone rubber, diene type liquid rubber, and the like.
The curable rubber polymers are classified into a room-temperature-curable type, a heat-curable type, and a UV- or electron-ray-curable type. Of these types, the UV- or electron-ray curable type of rubber can be cured in an extremely short time of from several seconds to several minutes, and exhibits significantly high productivity.
The UV- or electron-ray curable type of rubber polymer undergoes generally the cure reaction in a radical chain reaction caused by a radical formed by irradiation of a UV ray or an electron ray. Therefore, such problem is involved that a part of the radicals generated in the cure reaction will react with the organic coloring matter in the recording layer to deteriorate the recording layer.